Kaolinite is a Clay Mineral with the chemical composition Al2Si2O5(OH)4. It is a layered Silicate Material, with one Tetrahedral sheet linked through Oxygen Atoms to one Octahedral sheet of Alumina Octahedra. Rocks that are rich in kaolinite are known as china clay or kaolin.
The name is derived from Gaoling or Kao-Ling ("High Hill") in Jingdezhen, Jiangxi Province, China. Kaolinite was first described as a Mineral Species in 1867 for an occurrence in the Jari River basin of Brazil.
Kaolinite is one of the most common minerals; it is mined, as kaolin, in Brazil, France, United Kingdom, Germany, India, Australia, Korea, the People's Republic of China, the Czech Republic and the United States.
Kaolinite has a low Shrink Well- Capacity and a low Cation Exchange Capacity (1-15 meq/100g.) It is a soft, earthy, usually white mineral (dioctahedral phyllosilicate Clay), produced by the chemical weathering of aluminium silicate minerals like Feldspar. In many parts of the world, it is colored pink-orange-red by iron oxide, giving it a distinct Rust hue. Lighter concentrations yield white, yellow or light orange colours. Alternating layers are sometimes found, as at Providence Canyon State Park in Georgia, USA.
Predominance in tropical soils
Kaolinite clay occurs in abundance in soils that have formed from the chemical weathering of rocks in hot, moist climates - for example in tropical rainforest areas. Comparing soils along a gradient towards progressively cooler or drier climates, the proportion of kaolonite decreases, while the proportion of other clay minerals such as illite (in cooler climates) or smectite (in drier climates) increases. Such climatically-related differences in clay mineral content are often used to infer changes in climates in the geological past, where ancient soils have been buried and preserved.
Structural transformations
Kaolin-type clays undergo a series of phase transformations upon thermal treatment in air at atmospheric pressure. Endothermic dehydroxylation (or alternatively, dehydration) begins at 550-600 °C to produce disordered metakaolin, Al2Si2O7, but continuous hydroxyl loss (-OH) is observed up to 900 °C and has been attributed to gradual oxolation of the metakaolin (Bellotto et al., 1995). Due to historic disagreement concerning the nature of the metakaolin phase, extensive research has led to general consensus that metakaolin is not a simple mixture of amorphous silica (SiO2) and alumina (Al2O3), but rather a complex amorphous structure that retains some longer-range order (but not strictly crystalline) due to stacking of its hexagonal layers (Bellotto et al., 1995).
2 Al2Si2O5(OH)4 —> 2 Al2Si2O7 + 4 H2O
Further heating to 925-950 °C converts metakaolin to a defect aluminuim-silicon spinel, Si3Al4O12, which is sometimes also referred to as a gamma-alumina type structure:
2 Al2Si2O7 —> Si3Al4O12 + SiO2
Upon calcination to ~1050 °C, the spinel phase (Si3Al4O12) nucleates and transforms to mullite, 3 Al2O3 · 2 SiO2, and highly crystalline cristobalite, SiO2:
3 Si3Al4O12 —> 2 Si2Al6O13 + 5 SiO2
Uses
Kaolin is used in Ceramics, Medicine, Coated Paper as a Food Additive, in ToothPaste, as a light fusing material in white incandescent Light Bulbs, and in Cosmetics. It is generally the main component in Porcelain.
It is also used in Paint to extend titanium dioxide (TiO2) and modify gloss levels; in Ruber for semi-reinforcing properties and in adhesives to modify Rheology.
The largest use is in the production of paper, including ensuring the gloss on some grades of paper. Commercial grades of kaolin are supplied and transported as dry powder, semi-dry noodle or as liquid Slurry.
A more recent, and more limited, use is as a specially formulated spray applied to especially organic fruits, vegetables, and other vegetation to repel or deter insect damage, and at least in the case of apples, to prevent sun scald. A traditional use is to soothe an upset Stomach, similar to the way Parrots (and later, humans) in South America originally used it.
Kaolin is, or has been, used as the active substance in liquid anti-diarrhea medicines such as Kaomagma and Kaopectate. Such medicines were changed away from aluminium substances due to a scare over Alzheimer's Disease, but have since changed back to compounds containing aluminium as they are more effective. Kaolin is known in Traditional Chinese Medicine as an herb under the name 赤石脂 (chì shí zhī), "crimson stone resin" in a direct translation. It's taste is sweet, astringent and warm. In TCM it's used for restricting leakage from intestines and stopping diarrhea, blood containment and stopping bleeding, wounds healing.
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